1. Field of the Invention
The present invention relates to a transformer apparatus, and in particular, to a transformer apparatus for use in an insulated switching power supply apparatus for the purpose of reduction of switching noise flowing between a primarily side ground and a secondary side ground of the transformer apparatus.
2. Description of the Related Art
First of all, an impedance meter is cited as an conventional example using an insulated switching power supply apparatus, and then, the insulated switching power supply apparatus, in particular, a performance required for a transformer apparatus used therefor will be explained below. FIG. 7 is a circuit diagram showing an example of an impedance meter when one end of a test element is grounded, that is, in one-line grounded measurement. In the arrangement shown in FIG. 7, an ampere meter 73 has a ground point common to a ground point of a test element 70. However, it is necessary to separate an alternating current signal source 71 and a voltmeter 72 from the ground point of the test element 70. For this reason, an electric power must be supplied to the alternating current signal source 71 and the voltmeter 72 from the insulated switching power supply apparatus of DC-DC converter.
In the case of supplying an electric power to the alternating current signal source 71 and the voltmeter 72 from an insulated DC-DC converter 100, this leads to such a problem as a switching noise of the insulated DC-DC converter 100. As shown in FIG. 8, when a switching noise current flows between a primary side ground and a secondary side ground of the DC-DC converter 100, the switching noise current flows into the ampere meter 73 of the impedance meter, and then, this interferes an impedance measurement.
A method of evaluating the switching noise current flowing into the ampere meter 73 to give a quantitative index is shown in FIG. 9, paying attention to a performance of a single transformer apparatus, which is a part for determining a magnitude of switching noise. A transformer apparatus is connected in a manner as shown in FIG. 9, and then, an electrostatic capacitance value C is measured by using the following equation:
C=(I/V)xc3x97(1/jxcfx89)xe2x80x83xe2x80x83(1)
where I denotes a value measured by the ampere meter 93;
V denotes a value measured by the voltmeter 92; and
xcfx89 denotes an angular frequency (=2xcfx80f), where f denotes a signal frequency of a signal source 91.
The factor when a current flows between the primary side ground and the secondary side ground is not always electrostatic coupling between the primary side ground and the secondary side ground, and in many cases, it is due to a leakage magnetic flux. In any case, when a voltage of the primary side exciting the transformer apparatus is set to a constant value, there are many cases where a current flowing between the primary and secondary sides is proportional to the frequency, and then, it is convenient to express the amplitude of the noise current as an electrostatic capacitance value.
FIGS. 10 and 11 show a structure of a conventional transformer apparatus. Referring to FIGS. 10 and 11, coaxial cables are utilized as lead wires of a primary winding 121 and a secondary winding 122 so as not to generate a magnetic flux outside the transformer apparatus. These primary winding 121 and secondary winding 122 are subjected to electrostatic shield 115, and then, respective electrostatic capacitances C10 between the primary winding 121 and a secondary side ground 132 and between the secondary winding 122 and a primary side ground 131 are small. Accordingly, this is not a principal factor of switching noise current flowing via the primary side ground and the secondary side ground (See FIG. 12).
The problem is leakage magnetic fluxes 141 and 142 existing outside a core 110. A leakage magnetic flux crossing across a space is classified into two cases as shown in FIG. 13, that is, an inside of the transformer apparatus and an outside thereof. In the transformer apparatus, the leakage magnetic flux 142 crosses across the space formed by the primary side ground and the secondary side ground so as to generate an electromotive force, and then, a switching noise current flows though the electrostatic capacitance between the primary side ground and the secondary side ground. Moreover, it is possible to cancel the generated electromotive force depending upon a shape of the primary side ground or the secondary side ground, and a position of ground lead wire. However, in the conventional transformer apparatus, it is difficult to find out the optimal shape and position of the lead wire, and also, it is difficult to obtain a geometric reproducibility. Therefore, canceling effect by this method is low. On the other hand, on the outside of the transformer apparatus, the leakage magnetic flux 141 crosses across the space formed by the lead wire of the primary side ground, the lead wire of the secondary side ground and an ampere meter connecting both the grounds so as to generate an electromotive force, and this becomes a factor of generating a switching noise current.
When the electrostatic capacitance value C is measured according to the above method shown in FIG. 9, in the conventional transformer apparatus, the limit of electrostatic capacitance is about 200 fF. An influence will be described when the aforesaid conventional transformer apparatus is utilized for the DC-DC converter 100 of the impedance meter shown in FIG. 8. Assuming that the switching frequency of the DC-DC converter 100 is set to 200 kHz, and the voltage of the switching frequency component of a primary side voltage exciting the transformer apparatus is set as 12 Vrms, then a switching noise current flowing through the ampere meter shown in FIG. 8 is obtained by the following equation:
12Vrmsxc3x97(2xc3x97xcfx80xc3x97200kHzxc3x97200fF)≈3xcexcArmsxe2x80x83xe2x80x83(2).
In the case of measuring a 100 kxcexa9 resistance at a 100 mVrms signal by the impedance meter, a measurement signal flowing through the ampere meter 73 becomes 1 xcexcArms. Therefore, the above switching noise current of 3 xcexcArms is larger than that of the measurement signal. For this reason, it is impossible to avoid a saturation of the ampere meter by the switching noise current.
As described above, in the DC-DC converter 100 using the conventional transformer apparatus, a switching noise current generated by the DC-DC converter 100 becomes large, and then, it is difficult to high accurately measure a minute current.
An essential object of the present invention is accordingly to provide a transformer apparatus capable of reducing a switching noise current flowing between a primary side ground and a secondary side ground.
According to one aspect of the present invention, there is provided a transformer apparatus comprising:
a first core having a first primary winding wound around the first core;
a second core having a substantially same structure as that of the first core, and having a second primary winding wound around the second core, the second primary winding being connected in parallel to the first primary winding;
a first conductor housing;
a second conductor housing; and
a third core having a secondary winding around the third core, the third core being entirely and electrostatically shielded by the first conductor housing,
wherein the third core is arranged so as to be sandwiched between the first and second cores respectively via first and second electrostatically shielding disks electrically connected to the second conductor housing, and
wherein the first, second and third cores are electrically connected by the second conductor housing operating as one-turn winding, and are electrostatically and magnetically shielded from the outside of the transformer apparatus by the second conductor housing.
Therefore, according to the present invention, in the transformer apparatus, the primary side circuit is arranged in a form of symmetrical structure, and further, three cores are electrically coupled by one-turn winding provided by the second conductor housing, and are electrostatically and magnetically shielded from the outside by the second conductor housing. Accordingly, it is possible to substantially reduce a switching noise current of the DC-DC converter 100 as the conventional transformer apparatus. Therefore, it is possible to measure a minute current in an impedance meter without any interference caused by the switching noise current, and to more accurately measure an impedance as compared with the conventional case.
According to another aspect of the present invention, there is provided a transformer apparatus comprising:
first and second transformers having a substantially same structure as each other,
wherein the first transformer comprises:
a first core having a first primary winding wound around the first core;
a first conductor housing;
a second conductor housing; and
a third core having a first secondary winding wound around the third core, the third core being entirely and electrostatically shielded by the first conductor housing,
wherein the first and third cores are arranged via a first electrostatically shielding disk electrically connected to the second conductor housing, are electrically connected by the second conductor housing operating as one-turn winding, and are electrostatically and magnetically shielded from the outside of the transformer apparatus by the second conductor housing,
wherein the second transformer comprises:
a second core having a second primary winding wound around the second core, the second primary winding being connected in parallel to the first primary winding;
a third conductor housing;
a fourth conductor housing; and
a fourth core having a second secondary winding wound around the fourth core, the fourth secondary winding being connected in series to the first secondary winding, the fourth core being entirely and electrostatically shielded by the third conductor housing, and
wherein the second and fourth cores are arranged via a second electrostatically shielding disk electrically connected to a fourth conductor housing, are electrically connected by the fourth conductor housing operating as one-turn winding, and are electrostatically and magnetically shielded from the outside of the transformer apparatus by the fourth conductor housing.
Therefore, according to the present invention, two transformers are used, and this leads to that these respective transformers mutually cancel the switching noise currents. Then it is possible to prevent a switching noise current from flowing outside these two transformers. Moreover, one transformer apparatus is divided into two transformers, and this leads to that the structure of the transformer apparatus becomes simpler than that of prior art, then it becomes easy to manufacture the same transformer apparatus.
In the above-mentioned transformer apparatus, the first and second transformers are preferably apposed with each other.
Therefore, according to the present invention, in addition to the above-mentioned advantageous effects, two transformers are apposed with each other on a flat substrate. This leads to that the entire height of the transformer apparatus is reduced, and it is possible to improve a degree of freedom upon mounting the transformer apparatus to various equipments.